Figure 1. Relative Emissions for Two Paths
(one passes from top to bottom layer (plot with peaks) and the other stays on one side)

Abstract: Printed wiring board, PWB, signal paths must
often change layers in a board stackup. Under some conditions this can
cause problems. An emissions example is used to illustrate the conditions
where changing layers can cause problems.

Discussion: Routing paths
in a PWB often requires the paths to change layers to accomplish the
layout. For a four layer PWB, this usually means changing from the top
layer to the bottom layer of the board, the two middle layers being
power and ground. A four layer board is especially problematical
because generally the separation between the power and ground layers is
relatively large, on the order of 30 to 40 mils, compared to boards
with six or more layers.

Figure 1 depicts emissions from the board shown in Figure 2, the same one used for an ESD example in last month's April 2006 Technical Tidbit,
The paths on the board are about 30 cm in length. One stays on the same
side of the board while the other passes through to the other side for
about 10 cm. The two metal planes are about 30 mils apart and are
shorted together by the SMA connectors and at the 47 Ohm load
resistors. The paths are made of one insulated conductor made from 24
gauge twisted pair telephone wire. Since the original cable had a
differential impedance of 100 Ohms, one of the wires taped down to a
copper plane will have a characteristic impedance of 50 Ohms with
respect to the metal plane.

The data for Figure 1 was generated by exciting one path with the
tracking generator of a spectrum analyzer and attaching an emissions
measurement antenna to the input of the spectrum analyzer. The antenna had
reasonable antenna factors up to about 3 GHz. Figure 3 shows the board
sitting on a cardboard box in an EMC chamber. The coax on the left
comes from the tracking generator and the one meter white wire on the right
is just hanging off the box connected to one of the board planes by an alligator clip. It was included in case signal voltage on
the planes might cause attached cables to radiate. The coaxial cable
shield and the added wire form something of a dipole, albeit an
asymmetric one.

Figure 2. Test Board with Paths on a Single Layer and Two Layers

Figure 3. Emissions Test Setup

Figure 1 was generated by overlaying emissions data from the two
paths shown in Figure 2. The plots cover the frequency range from near
DC up to 2.9 GHz. The absolute magnitude of the plots is not important.
It depends on the output power of the tracking generator, but the
comparison between the two plots is striking. The lower trace resulted
from the path that stayed on one side of the board while the upper
trace, showing pronounced peaks, resulted when the path that passed
through the board was energized. Note that the first peak at 247 MHz is
nearly 30 dB above the emissions at that frequency from the path that
stayed on the same side of the board.

Figure 4 shows the signal currents in the wire and planes as the signal
passes through the boards. As described in detail in last month's April 2006 Technical Tidbit,
a complex interplane impedance, Z, appears in the signal return path as
it passes between the planes. The voltage generated across this
impedance by the signal current as well as other effects contribute to
the emissions shown in Figure 1.

Figure 4. Signal Paths Through a Via

For cases where the interplane distance is much smaller than 30 mils,
the interplane impedance will also generally be lower as well and the
effect shown in Figure 1 will be smaller and less of a problem. The
effect can also be minimized for a four layer board if critical signals
transition from the top layer to the bottom layer of the board near existing (for low cost)
bypass capacitors.

Summary:
Transitioning between layers of a PWB can introduce significant
impairment into a signal path. The larger the spacing between power and ground planes, the larger the
effect. The example of a "four layer" PWB emissions profile shows one of
the problems that can occur.

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